The old muffler probably had pieces in it from the cat due to the blown plenum. 98 is not a good year for plenum gasket issues.

 

how would I know if the plenum gasket is blown?

 

This is likely true. At least, at some point in time I'd imagine.

As for 98 not being a good year, neither is 94, 95, 96, 97, 99, 00, 01, or 02.
Read the End-all-Be-all Plenum thread that's stickied in the tech section.

 

well I have read it and I don't have any of those problems. Truck runs great now that the muffler is gone. it does use a little oil but not a great deal.

 

Just you wait, homeslice.

Your time in the sh1tstorm is coming.

 

My plenum is blown and I don't have symptoms (aside from the oil in the intake part). So just because you don't have symptoms yet doesn't mean its not blown.

 

good point! I guess I might as well go ahead and do a check!